Optical systems may be employed as projection objectives in projection exposure systems used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks (or reticles) onto an object having a photosensitive coating at ultrahigh resolution.
In order to allow creating even finer structures, various approaches to improving the resolving power of projection objectives are being pursued. It is well known that resolving power may be improved by increasing the image-side numerical aperture (NA) of the projection objective. Another approach is employing shorter-wavelength electromagnetic radiation.
Deep ultraviolet (DUV) lithography at 193 nm, for example, typically involves a projection system with a numerical aperture of 0.75 or higher to achieve 0.2 μm or smaller features. At this NA, the depth of focus (DOF) is some tenths of a micrometer. In addition, fabrication and assembly tolerances make it difficult to build optical systems with such as large NA.
As is known in the art, short wavelength ultraviolet radiation (less than about 193 nm) is not generally compatible with many refractive lens materials due to the intrinsic bulk absorption. To reduce the radiation absorption within an optical system, reflective elements may be used in place of refractive optical elements. State of the art DUV systems often use catadioptric optical systems which include refractive lenses and reflective elements (mirrors).
Systems that operate at moderate numerical apertures and improve resolving power largely by employing short-wavelength electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region have been developed. In the case of EUV-photolithography employing operating wavelengths of 13.5 nm, resolutions of the order of 0.1 μm at typical depths of focus of the order of 1 μm may theoretically be obtained for numerical apertures of NA=0.1.
It is well known that radiation from the extreme-ultraviolet spectral region cannot usually be focused using refractive optical elements, since radiation at the short wavelengths involved is usually absorbed by the known optical materials that are transparent at longer wavelengths. Pure mirror system (catoptric optical systems) that have several concavely and/or convexly curved mirrors that have reflective coatings are thus employed in EUV-photolithography. The reflective coatings employed are typically multilayer coatings having, for example, alternating lay-ers (films) of molybdenum and silicon.
In the manufacture of semiconductor components and other finely structured components, a pattern from a mask to be imaged on a substrate is usually formed by lines and other structural units representing a specific layer of the component to be produced. The structures to be produced for semiconductor components typically include tiny metallic tracks and silicon tracks as well as other structural elements, which may be characterized by critical dimensions (CD) which, in the case of EUV-photolithography may be in the order of 100 nm or below. Where the pattern of a mask has structural features with given critical dimension on different parts of the mask, it is desired to reproduce the relative dimensions as precisely as possible in the structured substrate. However, various influences involved in the lithography process may result in undesirable variations of the critical dimensions (CD variations) in the structured substrate, which may affect the performance of the structured components negatively. Therefore, it is generally desired to improve lithography equipment and processes to minimize CD variations, especially lateral variations across the exposed field.
In many applications, linear features of the pattern run in different directions. It has been observed that under certain conditions a contrast obtained in a lithographic process depends on the structural orientation, thereby leading to what is commonly denoted as horizontal-vertical differences (H-V differences), which may affect the performance of the structured components negatively. Therefore, it may be desired to improve lithographic equipment and processes to minimize H-V differences.
Photolithographic equipment, or steppers, employ two different methods for projecting a mask onto a substrate, namely, the “step-and-repeat” method and the “step-and-scan” method. In the case of the “step-and-repeat” method, large areas of the substrate are exposed in turn, using the entire pattern present on the reticle. The associated projection optics thus have an image field that is large enough to allow imaging the entire mask onto the substrate. The substrate is translated after each exposure and the exposure procedure repeated. In the case of the step-and-scan method that is preferred here, the pattern on the mask is scanned onto the substrate through a movable slit, where the mask and slit are synchronously translated in parallel directions at rates whose ratio equals the projection objectives magnification.